Recent years have seen the expansion of Bluetooth technology beyond being a standard feature in cell-phones and personal computers into diverse applications including IoT (Internet of Things) systems and devices such as wireless speakers and headphones, cars, wearables and medical devices. Bluetooth (BT) and Bluetooth Low Energy (BLE) are unsurpassed for use in devices and systems that need to wirelessly send short bursts of data over short distances.
A problem for conventional Bluetooth systems, which use a Gaussian frequency-shift keying (GFSK) modulation, is that a test specification established by Bluetooth Special Interest Group™ requires that any Bluetooth receiver be able to receive dirty packets (dirty transmitter or dirty Tx) as specified in either a BT or a BLE test specification. Briefly, as specified in the BT/BLE test standards, in dirty packets, a modulation index keeps changing quickly over time from one dirty packet to the next. For BT basic data rate (BDR), the modulation index of dirty packets jumps up/down for every 20 ms or 16 packets. For Bluetooth Low Energy (BLE) the modulation index of dirty packets jumps up/down for every 50 packets. Besides modulation index changes, carrier frequency offset and symbol timing error are also introduced in the dirty transmitter profile to construct non-ideal dirty signals used in the test, which are within specification limits but deviate from the ideal case.
One possible solution is the use of a GFSK demodulator configured to use maximum likelihood sequence estimation (MLSE) algorithm, which can provide improved received sensitivity. The drawback of a GFSK demodulator using MLSE is that it requires a very accurate estimation of modulation index, typically within ±2%, to achieve any improvement in sensitivity over a conventional, non-MLSE demodulator, and the test specification does not have stringent requirement on GFSK modulation index from transmitter side. For BDR, the required modulation index range is from 0.28 to 0.35. For BLE systems, the required modulation index range is from 0.45 to 0.55. Thus, a modulation index estimation circuity is needed for MLSE demodulator. However, the per-packet estimates of the modulation index are not accurate, especially near a sensitivity threshold of the receiver, due to a short length of the training sequence. Moreover, as noted above in dirty packets or dirty TX, the modulation index keeps changing quickly over time. Thus, in cases where the increased sensitivity of MLSE is needed most, i.e., for dirty packets, it produces results less accurate than for conventional, non-MLSE enabled demodulators.
Accordingly, there is a need for a communication system and method of operating the same to improve receive sensitivity that is not limited by the need for an accurate estimation of the modulation index.